1. Field of the Invention
The present invention relates to a frequency discrimination circuit which includes a monostable circuit into which an input frequency signal is fed, and a D flip-flop which is connected to the output side of the monostable circuit, for maintaining an output signal, which discriminates the high and low levels of the frequency signal which is fed into the monostable circuit, from the D flip-flop.
2. Description of the Prior Art
A conventional frequency discrimination circuit, is illustrated in FIG. 1 (for example, as shown on page 633 of the "Hitachi KTL Data Book," published in March 1983). This circuit comprises an input signal source 1, a monostable circuit 2 into which a frequency pulse signal is fed from the input signal source 1, a D flip-flop 3 which receives, at the same time, the output signal of the monostable circuit 2 into its input terminal D and the frequency signal from the input signal source 1 into its clock input terminal CK, and an output terminal 4 which receives a frequency discriminated signal from a positive output terminal Q2 of the D flip-flop 3.
The pulse signal P, whose frequency fluctuates as shown in FIG. 2 (A), is outputed from the input signal source 1. When the D flip-flop 3 is put into reset status, the monostable circuit 2, as shown in FIG. 2 (B), is triggered at the trailing edge of the pulse signal P, and outputs the signal M, which intermittently reaches its high level for a prescribed time To, according to the prescribed time constant.
A D flip-flop 3 outputs the frequency discriminated signal F from its positive output terminal Q according to the pulse signal P from the input signal source 1 and the output signal M of the monostable circuit 2, and at the point where the ouptut signal M of the monostable circuit 2 is at the high level, and where the pulse signal P of the input signal source 1 becomes the high level, as shown in FIG. 2 (C). Then, at the point where the output signal M reaches its low level and the pulse signal P reaches its high level, the frequency discriminated signal returns to its low level. Accordingly, because the frequency discriminated signal F from the D flip-flop 3 reaches the high level at the time when the frequency of the pulse signal PO from the input signal source 1 reaches a high frequency exceeding the prescribed frequency, it is possible to discriminate the high and low levels of the input frequency signal according to the frequency discriminated signal F.
However, in this type of conventional frequency discrimination circuit, the time constant of the monostable circuit 2 is constant, so for this reason, at the time when the pulse width of that output signal P rises to the neighborhood of the set frequency, the frequency discriminated signal F of the D flip-flop produces troublesome hunting.